Coal washing process



July 17, 1956 Filed March 2, 1954 FIG. I

c. KRIJGSMAN ET AL 2,754,963

COAL WASHING PROCESS 2 Sheets-Sheet l July 17, 1956, V c, KRUGSMAN ET AL2,754,963

com. WASHING PROCESS Filed March 2, 1954 k 2 Sheets-Sheet 2 UnitedStates Patent ()fifice COAL WASHING PROCESS Centinus Krijgsman,Hoensbroek, Freerk J. Fontein,

Heerlen, and Jan N. J. Leeman, Brunssurn, Netherlands, assignors toStamicarbon N. V., Heerlen, Netherlands Application March 2, 1954,Serial No. 413,562

11 Claims. (Cl. 20912) This invention relates to coal washing. More inparticular this invention relates to a heavy media process of coalwashing by means of a suspension of fine shale particles and other finewaste particles derived from the raw coal.

It is the object of this invention to provide an improved processwherein the separation is sharp and easily adaptable to varyingconditions, wherein the various machines are efliciently used and waterconsumption is maintained at a minimum. It is in particular the objectof the present invention to provide a process in which the amount ofliquid in the circuit and the level of the suspension in the variousmachines is readily kept constant, and wherein the specific gravityaccording to which the separation is made can easily be keptsubstantially constant at a desired value.

in summary these and other objects are obtained by feeding the coal to awashing tank which contains a suspension in water of fine shaleparticles or other fine waste particles derived from the raw coal;separately removing the floating and the settled particles from thesuspension; feeding the particles thus removed to washing screens;spraying these particles first with clarified liquid from asuspension-thickener and thereafter with water; screening the underflowfrom the washing screens on a fine screen with apertures smaller than0.6 mm. so as to remove coal and shale particles which are too coarse tobe efficiently separated by a froth-flotation process; subjecting theundersize from the fine screen to a froth-flptation process in afroth-flotation machine; withdrawing underflow from the froth-flotationmachine at a larger rate than is introduced therein as underflow fromthe fine screen and feeding such underflow to the saidsuspensionthickener; feeding thickened suspension from thesuspension-thickener to the washing tank; feeding clarified liquid fromthe suspension-thickener to an overflow-vessel; feeding clarified liquidfrom the overflow-vessel to the frothflotation machine at a rate so asto maintain the level of the liquid in the froth-flotation machinesubstantially constant; and removing the balance of the feed to theoverflow-vessel from the circuit.

The process and apparatus adapted to be used in this process will befurther explained with reference to the drawings, of which Figure 1schematically shows a flow-sheet of a coal washing plant according tothe invention, and

Figure 2 schematically shows means for controlling the level and thespecific gravity of the suspension in two washing tanks which arearranged in series.

In Figure l raw coal, larger than for instance mm. is fed to a washingtank 1, which contains a suspension convenient manner.

A frothing agent, such as pine oil, is introduced in the of fine wasteparticles derived from the coal, such as shale, pyrite and sand,hereinafter referred to as shale. The specific gravity of thissuspension is such that coal floats thereon and that shale, which has ahigher specific gravity than coal, sinks. This is the reason that thiskind of separation is known as the sink-and-float process.

The Washing tank 1 is provided with a conventional device, such as anendless scraper, which removes the separated particles from thesuspension and feeds the sink product, that is the shale fraction, to aWashingscreen 2, and the float product, that is the coal fraction, to ascreen 3. Above screen 2 are sprayheads 4 which are fed via a pipe 5with clarified liquid, and sprayheads 6 which are fed with water via apipe 7. On the screen 2 suspension adhering to the shale particles isremoved so that clean shale is discharged from the screen 2. Theundersize from the screen 2 consists of diluted suspension which is fedvia a pipe 8 to a screen 9.

Above screen 3 are sprayheads 10 which are fed via a pipe 11 withclarified liquid and sprayheads 12 which are fed with water via a pipe13. The oversize from screen 3 consists of clean coal which is removedfrom the circuit; the undersize consists of diluted suspension which isfed via pipe 8 to screen 9. i V

The undersize from screen 9 is fed to a froth-flotation machine 14 via afeed pipe 15 and the function of screen 9 is to prevent particles whichare too coarse for frothflotation from entering the froth-flotationmachine 14. Therefore screen 9 has apertures of at most 0.6 mm. andpreferably of about 0.5 mm. Width. The oversize from screen 9 is removedfrom the circuit; it may be discarded or be further treated in a washingplant for fine coal or it can be mixed with clean coal or be used in anyfeed pipe 15. In the froth-flotation machine 14 shale particles settlebut coal particles are caught in the froth which floats. The coalbearingfroth is removed from the machine and from the circuit.

A diluted shale suspension is discharged from the frothflotation machine14 through a discharge aperture 16 and is pumped by a pump 17 via a pipe18 to a hydrocyclone-thickener 19. Thickened suspension is dischargedtherefrom through its apex aperture 2% and is fed via a pipe 21 andseveral devices which will be discussed later to washing tank 1.

23 and pipes 5 and 11 to the sprayheads 4 and 10.

both coal and shale and which are hereinafter referred to as mixed, andshale particles. In broad terms it is the task of the washing plant toseparate the coal from the shale, but it will be clear that at leastmixed with a high coal content should also be separated from the shaleand should be either obtained separately or as part of the coalfraction. Likewise mixed with a high ash content should be eitherobtained as a separate fraction or as part of the shale fraction. Oftenthree fractions are made, called coal-, mixedand shale fraction, but

it should be understood that the coaland shale fractions 7 also containmixed.

Mixed with a low coal content has a higher specific gravity than coal ormixed with a high coal content and Patented July 17, 1956 Clarifiedliquid is discharged through the overflow aperture 22 of thehydrocyclone-thickener 19 and flows via a flow divider it is thereforepossible to separate the raw coal into fractions according to the coalcontent of the particles.

Figure 2 illustrates a variation of an installation of Figure 1, adaptedfor separation into three products and like numerals are employedtherein to indicate like elements. According to Figure 2 there are twowashing tanks 1 and 24. The suspension in washing tank 1 has a higherspecific gravity than the suspension in washing tank 24; the specificgravity in washing tank 1 may for instance be 1.6 and in washing tank 241.4.

The shale fraction from washing tank 1 is fed to screen 2; the floatproduct from washing tank 1, consisting of coal and mixed, is fed towashing tank 24 and separated therein. The coal fraction is dischargedto washing screen 3 and the mixed fraction is discharged to a washingscreen 25.

Above screen 25 are sprayheads 26 which are fed via a pipe 27 withclarified liquid from flow divider 23, and sprayheads 28 which are fedwith water via a pipe 29. Clean mixed is discharged from screen 25 andis removed from the circuit. The undersize from screen 25 consists ofdiluted suspension which is united with the undersize from screens 2 and3.

It is important that the specific gravity of the suspension in everywashing tank is accurately controlled. In general the specific gravityshould be kept constant, but it may be necessary to change the qualityof the produced coal or mixed, or the quality of the raw coal may changeand in such cases the specific gravity of the suspension in the washingtanks must be changed and maintained at the new level.

Figure 2 schematically shows how the specific gravity in the washingtanks 1 and 24 is controlled and also how the level of the suspension inthe washing tanks is controlled.

The specific gravity of the suspension in washing tank 1 isautomatically controlled by means of a pressure sensitive instrument 30,for instance a Foxboro M 40 Stabilog Controller of the Bell type with areset. The instrument 30 is connected with bubble tubes 31 and 32 whichhave open ends terminating in vertically spaced relation in thesuspension in washing tank 1. Compressed air is introduced through pipe33 to instrument 30, through pipes 33 and 34 and reducing valve 35 tobubble tube 31 and through pipe 36 and reducing valve 37 to bubble tube32. In bubble tube 31, which terminates not as deep in the suspension asbubble tube 32, there is a closed bubble vessel 38 which contains someliquid. The air delivered to the bubble tubes 31 and 32 will encounterdifferent pressureresistance in washing tank 1 due to the verticalspacing of their ends. The pressure difference between the tubes 31 and32 thus depends on the specific gravity of the suspension in washingtank 1. The instrument 30 responds to the pressure difference betweenthe tubes 31 and 32 and consequently to the specific gravity in washingtank 1, so that if the specific gravity in washing tank 1 decreases, thepressure in the pipe 39, which is connected with the instrument 30,increases (This arrangement has been described in more detail in U. S.Patent 2,649,963 and in the Journal of the Institute of Fuel, April1948, page 192.) The pipe 39 is connected through pipe 49 and valve 41with a device 42 which controls the diameter of the apex aperture 20 ofhydrocyclone 19. As has been described in more detail in U. S. Patents2,654,479 and 2,649,963, the specific gravity of the thickenedsuspension from hydrocyclone-thickener 19 can be controlled in thismanner: if the pressure in pipe increases the diameter of the apexaperture 20 decreases and the specific gravity of the apex fractionincreases. Consequently any deviation from the specific gravity to whichthe instrument 30 is adjusted is automatically counteracted until theproper specific gravity is restored.

It is also possible to use a hydrocyclone-thickener 19 arranged asdescribed in U. S. patent application Serial No. 360,464, filed June 9,1953 by Ian N. I. Leeman and Freerk I. Fontein, wherein the dischargeapertures of the hydrocyclone are provided with suction pipesdischarging into receivers under the liquid level therein. Under suchcircumstances the rate of apex discharge is a function of the weight ofthe liquid columns in the two suction pipes. By properly selecting thelength of those columns the hydrocyclone can be made to discharge athickened suspension the specific gravity of which is substantiallyconstant, even if the concentration of the feed varies. If such ahydrocyclone-thickener is used in the present process it should beregulated so as to discharge a suspension the specific gravity of whichis somewhat higher than the desired specific gravity of the suspensionin the washing tank 1. The proper specific gravity of the suspension inwashing tank 1 may then be attained by continuously feeding a controlledamount of clarified liquid from flow divider 23 via pipe 43 to washingtank 1. The quantity is regulated by means of valve 44 in pipe 43, valve44 being automatically adjusted by instrument 30 via pipe 39 and valve45 therein. When the pressure in pipe 39 decreases the valve 44 isopened or opened further. To prevent obstructions valve 44 preferably isa needle valve which is directly connected to flow-divider 23. It is ofcourse also possible to feed a controlled amount of water to washingtank 1 instead of clarified liquid.

It will be clear that either valve 41 or valve 45 should be opened. Ifthe hydrocyclone-thickener 19 is of the type described in U. S. Patent2,649,963 valve 45 is closed and the specific gravity is controlled bymeans of the device 42; if the hydrocyclone-thickener 19 is of the typedescribed in U. S. application Serial No. 360,464 the specific gravityis controlled by means of valve 44.

A small amount of shale which is used in the separating suspension iscontinuously lost; shale adheres to the solids removed from the circuitand as will be discussed in more detail hereinafter, also shalecontaining liquid is removed from the circuit. Consequently new fineshale must be supplied. To this end a portion of the clean shaledischarged from washing screen 2 is fed together with water or withclarified liquid from flow-divider 23 to a mill 46 (Figure l), forinstance a ball mill. The ground shale is classified in a classifier 47,the coarse fraction from which is returned through pipe 48 to the mill46, whereas the fines from classifier 47 are led through pipe 49 toscreen 9 and from there to the froth-flotation machine 14, but may alsobe directed immediately from classifier 47 to the froth-flotationmachine 14.

The capacity of the mill 46 and the classifier 47 which form a closedcircuit, is so large that normally they need not operate continuously.That is, if the mill-classifier unit is in operation, the amount of fineshale in the circuit increases, whereas otherwise this amount decreases.This is one of the reasons why a storage vessel 50 may be provided inthe circuit. As best shown in Figure 2, the thickened suspension fromhydrocyclone-thickener 19 is received in a pivoting funnel 51 which,under normal operating conditions, directs the suspension to acompartment 52 of receiver 53 from where the suspension fiOWs to aflow-divider 54. From flow-divider 54 one portion of the thickenedsuspension goes via a pipe 55 directly to washing tank 1, whereasanother portion is fed via a pipe 56 to storage vessel 50. The quantityfed to washing tank 1 via pipe 55 is made smaller than the quantitywhich must be continuously supplied to washing tank 1, and the balanceis supplied from storage tank 50 via a pipe 57. The quantity suppliedfrom storage tank 50 to washing tank 1 is regulated by means of valve58, preferably a needle valve at the infced end of pipe 57 so as toprevent clogging. Valve 58 is pneumatically operated by means of apressure sensitive instrument 59, for instance a Foxboro M 40 StabilogController of the Bellow type with a reset. This instrument 59 isconnected with a bubble tube 60 which has an open end terminating in thesuspension in washing tank 1. Compressed air is introduced through pipes61 and 62 to instrument 59 and through pipe 61 and reducing valve 63 tobubble tube 60. The pressure in bubble tube 60 depends on the level ofsuspension in washing tank 1. When this level drops too low the pressurein bubble tube 60 becomes too low and the instrument 59 reduces the airpressure in the pipe 64, which is connected with the instrument, as aresult of which the valve 58 is opened or is opened further, so that thelevel of the suspension in the washing tank 1 rises.

When the mill-classifier circuits 46 and 47 is in operation the level ofthe liquid in storage vessel 50 rises. When storage vessel 50 is filledthe mill-classifier circuits 46 and 47 is stopped until the storagevessel 50 is almost empty.

To prevent settling of the suspension in storage vessel 5% compressedair is introduced therein near the bottom via a pipe 65.

There is no reason for the level of the suspension in washing tank 1 tobecome too high. However, if accidentally too much liquid is introducedinto washing tank 1 the excess flows off through an overflow aperture 66and flows via a pipe 67 to pipe 8.

The above explains how the specific gravity and the level of thesuspension in the washing tank 1 are kept substantially constant. Thespecific gravity of the suspension can be controlled by selecting thelevel of the liquid in bubble vessel 38. Furthermore the instrument 30may be provided with means for regulating the relation between thepressure difference between the bubble tubes 31 and 32 on the one handand the pressure in pipe 39 on the other hand. A Foxboro M 40 StabilogController is provided with such means.

As has been mentioned already, the coal and the mixed are separated inwashing tank 24 and the suspension therein consequently has a lowerspecific gravity than the suspension in washing tank 1. Washing tank 24receives from washing tank 1 coal and mixed with adhering suspension. Tomaintain the desired specific gravity of the suspension in washing tank24 clarified liquid is supplied from flow-divider 23 via pipe 68 andvalve 69, preferably a needle valve. To control the specific gravity ofthe suspension in washing tank 24 the amount of clarified liquid fed towashing tank 24 is regulated. To this end there is a pressure sensitiveinstrument '70 which controls the specific gravity of the suspension inwashing tank 24 in substantially the same way wherein the instrument 30controls the specific gravity of the suspension in washing tank 1. Thereare bubble tubes '71 and 72 which have open ends terminating invertically spaced relation in the suspension in washing tank 24,reducing valves 73 and 74, pipes 75, 76, 77 and 78 and a bubble vessel79; instrument 70 regulates the pressure in pipe 78 in dependence on thepressure difference between the bubble tubes 71 and 72. When thispressure difference increases the pressure in pipe 78 decreases andvalve 69 is opened further.

Generally it is not necessary to feed thickened suspension into washingtank 24. If it is desired, however, to increase the specific gravity ofthe suspension in washing tank 24 thickened suspension may be withdrawnfrom storage vessel 5% via pipe 80 and valve 81 therein. Valve 81 may behand operated.

The amount of liquid introduced into washing tank 24 generally is largerthan the quantity of liquid which adheres to the particles dischargedfrom the washing tank. The excess liquid is removed through overflowaperture 85 and flows via pipe 86 to pipe 8.

According to Figure 2 the raw coal is first separated according to ahigher specific gravity, for instance 1.6, in washing tank 1 andthereafter according to a lower specific gravity, for instance 1.4, inwashing tank 24. This system is used when the raw coal is substantiallydry. If the raw coal contains an appreciable amount of water, it isgenerally preferred to feed the raw coal to the washing tank in whichthe suspension has the lower specific gravity (1.4) and to feed theshale and mixed discharged therefrom to the washing tank in which thesuspension has the higher specific gravity (1.6). In that case bothwashing tanks must continuously receive thickened suspension, the firstwashing tank in which the specific gravity is lowest for counteractingdilution by the water in the raw product and the second washing tankwhich holds the suspension with the higher specific gravity so as tocounteract dilution by the suspension adhering to the discharge from thefirst washing tank. Control of the level of the suspension in thewashing tanks is unnecessary, as both tanks will continuously overflow.

Thus in case of wet feed instrument 59 can be deleted and instruments 30and 70 are either made to control the quantities of thickened suspensionintroduced into the washing tank, or one of the instruments 30 and 70 ismade to control the apex aperture of the hydrocyclone thickener 16, theother instrument controlling the quantity of thickened suspensionintroduced into the appertaining washing tank.

Thus the system shown in Figure 2 can be readily adapted to the handlingof wet feed.

When there is but a single washing tank as in Figure l and the feed iswet, no clarified liquid has to be added to the washing tank 1. Thequantity of thickened suspension fed to the washing tank is thencontrolled so as to maintain the desired specific gravity in the washingtank 1, which should continuously overflow. In that case the specificgravity of the thickened suspension discharged from suspension thickener19 should of course be higher than the specific gravity of thesuspension in washing tank 1. 1

During operation water is continuously introduced into the system, viz.on the washing screens 2 and 3 or 2, 3 and 25. This makes it necessaryalso to withdraw liquid from the system continuously. To this end aquantity of clarified liquid is fed from flow-divider 23 to a pipe 87via a valve 88. The quantity of liquid fed to pipe 8'7 is made largerthan the quantity to be withdrawn from the circuit.

There is of course liquid adhering to the solids withdrawn from thecircuit, viz. the cleaned shale, mixed, coal, oversize from screen 9 andfroth from froth-flotation machine 14; on the other hand the raw coalmay be wet. By the quantity of liquid to be withdrawn from the circuitis therefore understood the quantity of liquid which must be removedfrom the circuit as a separate stream for maintaining a substantiallyconstant quantity of liquid in the circuit. Generally, the quantity ofliquid to be removed from the circuit is smaller than the quantity ofspraying water added on screens 2, 3 and 25. Furthermore it will makesome difference whether the millclassifier circuit 46 and 47 is inoperation or not.

Referring again to Figure l, the quantity of liquid fed to pipe 87 ismade larger than the quantity to be withdrawn from the circuit. Pipe 87discharges into an overflow vessel 89 which has an adjustable overflowrim 90. The overflow vessel 89 communicates with froth-flotation machine14 via a pipe 91, and the overflow rim 90 is so adjusted that the liquidin froth-flotation machine 14 is at the proper level, that is so high,that froth can be readily removed therefrom but not so high that asignificant amount of liquid is removed with the froth.

The capacity of pump 17 is such that the quantity of diluted suspensionwhich is discharged from the frothflotation machine 14 through dischargeaperture 16 is larger than the amount of liquid fed to thefroth-flotation machine through pipe 15 minus the liquid in the froth.As a consequence a portion of the liquid fed to overflow vessel 89 flowsthrough pipe 91 to froth-flotation machine 14 and the balance isdischarged over overflow rim 90. Consequently the excess of liquiddischarged through discharge aperture 16 is returned to thefroth-flotation machine through pipe 91 whereas the proper amount ofliquid is withdrawn from the circuit.

One advantage of this system is that as a consequence of the downwardcurrent which is maintained in pipe 91 only clarified liquid iswithdrawn from the circuit, and not suspension from froth-flotationmachine 14, which contains more shale particles than the clarifiedliquid. (It will be clear that these shale particles must be kept in thesystem, since all shale particles which are lost must be replaced by newshale particles from the mill-classifier circuit 46 and 47.)

Another advantage is that the liquid level in frothfiotation machine 14is automatically kept constant.

These two problems which here are solved together, give rise todifiiculties in other washing systems of the same kind, that is in oldsystems only one of these problems can be adequately solved.

The froth-flotation machine 1"! is preferably of the Kleinbentink typeas described in more detail in U. S. Patent 2,569,141, but other typesmay also be used.

In the above a few times reference has been made to normal operatingconditions. The meaning of this term is that all machines operatesatisfactorily and that there is a continuous supply of raw coal andwater. A plant must of course be able to cope with small interruptions.In case of serious breakdowns it will always be necessary to stop theplant, but in that case it still is important that operation can beeasily started again as soon as the necessary repairs have been made.Also, when the plant is shut-down for the night or for holidays,starting must be simple.

The separating suspension for instance tends to settle during ashut-down, thus clogging pipes and making starting of the washing tank 1hazardous. To cope with these problems there is a storage tank 92, seeFigure 1. During a shut-down all suspension is stored therein, being fedthereinto via washing tank 1, valve 93 and pipe 94. Compressed air isadmitted continuously during a shutdown or before restarting the plantto storage tank 92 at the bottom, so that when the plant is startedagain the suspension is ready for use. When starting again thesuspension is pumped via valve 95, pipe 96, pump 97 and pipe 98 towashing tank 1. The suspension which remains in pipes 96 and 98, pump 97and storage tank 92 is drained over valve 99. If desired this drainedsuspension can be collected in a bucket and be reintroduced into thecircuit or it can be discarded.

If there are two washing tanks 1 and 24 there is a received on thescreens 2 and 3 (and screen 25 of Figure l 2), so that the feed tohydrocyclone-thickencr 19 becomes very diluted. Under such circumstanceshydrocyclonethickener 19 cannot discharge a suspension of the desire lspecific gravity. To prevent dilution of the suspension in the washingtanks pivoting funnel 51 is under such circumstances set for discharginginto compartment 166 of receiver 53 and the apex discharge fromhydrocyclonethickener 19 is then returned from compartment Hi l via.pipe 101 to pipe 3, screen 9 and froth-flotation machine 14. It may beadvisable, however, to direct suspension received in compartment 100directly to froth-flotation machine 14.

Pivoting funnel 51 may be hand operated or be automatically controlled,for instance as described in U. S. Patent 2,649,963.

Furthermore it will be clear that there may be spare pumps and sparehydrocyclones available. It further is obvious that any number ofmachines may be employed in parallel to have a suthcient capacity, butin the drawings but one unit of each machine has been shown. A singlehydrocyclone-thickener for instance will in general be insufiicient.

By way of example the following figures are given for a plant accordingto Figure 1 with a capacity of 160 metric tons of raw coal per hour, thesize of the coal being between A1" and 4". The D. S. M. washing tank 1is 6'6 wide. Screens 2, 3 and 9 are Allis- Chalmers Low-Head vibratingscreens which are 5 wide and 10 long. The slotted apertures of screens 2and 3 nr /s" wide, the slotted apertures of screen 9 are 0.5 1112 1.wide. The Kleinbcntink froth-tlotation machine 14 has a diameter of 9'.There are 3 hydrocyclonethickener-s 19 operated at a feed pressure of 19p. s. i. These hydroeyclones have a diameter of 14''; the diameter ofthe feed aperture and of the overflow aperture is 2.; the diameter ofthe apex aperture can be varied between and 1% in the manner describedin U. 5. Patent 2,649,963; the cone angle is 20; the hydrocyclones havea cylindrical portion which is 4" high, and a vortex finder which is 4long. Ball mill 46 has a capacity of metric tons of shale per hour. Theoverflow from classifier 47 is for 95% finer than 60 mesh. Storagevessel has a capacity of 1500 U. S. gallons; storage tank 92 has acapacity of 5500 U. S. gallons. The capacity of pump 17 is 450 U. 5.gallons per minute, at the necessary pressure. The apex discharge fromthe hydrocyclone amounts to approximately 50 U. S. gallons per minute.

We claim:

1. A continuous process of washing coal comprising the steps ofseparating raw coal into coal and shale fractions by scans of aseparating suspension of fine shale particles in water, spraying each ofsaid separated fractions on washing screens first with clarified liquidand then with water, subjecting the underfiow from said washing screensto froth-flotation, withdrawing undertlow from the froth-flotationoperation at a rate exceeding the rate of washing screen underflow feedthereto, subjecting said froth-flotation underfiow to a thickeningoperation, feeding thickened suspension from the thickening operation tosaid sink-and-fioat suspension, utilizing part of the clarified liquidfrom said thickening operation for spraying said coal and shalefractions, and feeding part of said clarified liquid to saidfroth-flotation operation at a rate effective to maintain the liquidlevel therein substantially constant.

2. A process as defined in claim 1, wherein shale scparatcd from the rawcoal is subjected to closed circuit grinding and classification, and theoverflow from the classification step is fed to the froth-flotationoperation.

3. A process as defined in claim 2, wherein at least part of thethickened suspension from the thickening operation is stored prior tofeeding to said sink-and-fiout suspension, and said grinding operationis discontinuous, being started and stopped in accordance with thequantity of thickened suspension in storage.

4. A process as defined in claim 1, wherein the underflow from saidwashing screens, prior to froth-flotation, is finely screened to removetherefrom particles too course for etficient froth-flotation separation.

5. A process as defined in claim 1, wherein the thickening operation iscarried out in a hydrocyclonc adjusted to discharge a. thickenedsuspension of specific gravity higher than th desired specific gravityof said separating suspension.

6. A process as defined in claim 1, wherein the specific gravity of saidseparating suspension is maintained constant by feeding thereto anautomatically controlled quantity of clarified liquid from saidthickening operation.

7. A process as defined in claim 1, wherein the specific gravity of saidseparating suspension is maintained constant by automaticallycontrolling the specific gravity of thickened suspension from thethickening operation.

8. A process as defined in claim 1, wherein the raw coal is wet and thespecific gravity of said separating suspension is maintained constant byfeeding thereto an automatically controlled quantity of thickenedsuspension from the thickening operation.

9. A process as defined in claim 1, wherein the level of said separatingsuspension is maintained constant by I 9 automatically controlling thefeed thereto of thickened suspension from the thickening operation.

10. A process as defined in claim 1, wherein the coal fraction isseparated into fractions in a second separating suspension of lowerspecific gravity than the first separat ing suspension.

11. A process as defined in claim 1, wherein the coal fraction isseparated into fractions in a second separating suspension of lowerspecific gravity than the first sinkand-float suspension, and thespecific gravity of said second separating suspension is maintainedconstant by feed- 10 ing thereto an automatically controlled quantity ofclarified liquid from said thickening operation.

References Cited in the file of this patent UNITED STATES PATENTS1,656,270 Downs et a1 Jan. 17, 1928 2,113,609 Wuensch Apr. 12, 19382,497,790 Pauvrasseau Feb. 14, 1950 2,569,141 Bakels Sept. 25, 19512,649,963 Fontein Aug. 25, 1953

1. A CONTINUOUS PROCESS OF WASHING COAL COMPRISING THE STEPS OFSEPARATING RAW COAL INTO COAL AND SHALE FRACTIONS BY MEANS OF ASEPARATING SUSPENSION OF FINE SHALE PARTICLES IN WATER, SPRAYING EACH OFSAID SEPARATED FRACTIONS ON WASHING SCREENS FIRST WITH CLARIFIED LIQUIDAND THEN WITH WATER, SUBJECTING THE UNDERFLOW FROM SAID WASHING SCREENSTO FROTH-FLOTATION, WITHDRAWING UNDERFLOW FROM THE FROTH-FLOTATIONOPERATION AT A RATE EXCEEDING THE RATE OF WASHING SCREEN UNDERFLOW FEEDTHERETO, SUBJECTING SAID FROTH-FLOTATION UNDERFLOW FEED THERETO,SUBJECTING SAID ING THICKENED SUSPENSION FROM THE THICKENING OPERATIONTO SAID SINK-AND-FLOAT SUSPENSION, UTILIZING PART OF THE CLARIFIEDLIQUID FROM SAID THICKENING OPERATION FOR SPRAYING SAID COAL AND SHALEFRACTIONS, AND FEEDING PART OF SAID CLARIFIED LIQUID TO SAIDFROTH-FLOTATION OPERATION AT A RATE EFFECTIVE TO MAINTAIN THE LIQUIDLEVEL THEREIN SUBSTNATIALLY CONSTANT.